1. Field of the Invention
The invention in general relates to the fabrication of layered superlattice materials, and more particularly to a chemical vapor deposition method for making thin films of layered superlattice materials, and to thin films of layered superlattice materials fabricated using such method.
2. Statement of the Problem
Ferroelectric compounds possess favorable characteristics for use in nonvolatile integrated circuit memories. See U.S. Pat. No. 5,046,043, issued Sep. 3, 1991, to Miller et al. A ferroelectric device, such as a capacitor, is useful as a nonvolatile memory when it possesses desired electronic characteristics, such as high residual polarization, good coercive field, high fatigue resistance, and low leakage current. Layered superlattice material oxides have been studied for use in integrated circuits. Layered superlattice materials exhibit characteristics in ferroelectric memories that are orders of magnitude superior to those of PZT and PLZT compounds.
U.S. Pat. No. 5,519,234, issued May 21, 1996, to Paz de Araujo et al., discloses that layered superlattice compounds, such as strontium bismuth tantalate (SBT), have excellent properties in ferroelectric applications as compared to the best prior materials and have high dielectric constants and low leakage currents. U.S. Pat. No. 5,434,102, issued Jul. 18, 1995, to Watanabe et al., and U.S. Pat. No. 5,468,684, issued Nov. 21, 1995, to Yoshimori et al., describe processes for integrating these materials into practical integrated circuits. Ferroelectric layered superlattice materials, like the metal oxides SrBi2Ta2O9 (SBT) and SrBi2(Ta1xe2x88x92xNbx)2O9 (SBTN), where 0xe2x89xa6xxe2x89xa61, are currently under development for use as capacitor dielectric in nonvolatile memory applications, such as in FeRAMs and nondestructible read-out ferroelectric FETs.
The layered superlattice materials currently being considered for use are metal oxides. Thin films of layered superlattice materials may be formed on a substrate using one of a number of techniques known in the art. Conventional deposition techniques include, for example, sputtering, sol-gel or metal organic deposition (xe2x80x9cMODxe2x80x9d) techniques. The MOD techniques are preferred for various reasons, including stability of liquid precursor solutions, and good control of stoichiometry in the precursor streams and in the ferroelectric thin film. An MOD technique for depositing a thin film of superlattice material typically entails the decomposition of metal organic precursors to form a solid film of metal oxide compounds on an integrated circuit substrate, and the reaction and crystallization of the metal oxide compounds in the solid film to form the desired polycrystalline layered superlattice material. Formation of polycrystalline layered superlattice material is necessary in order to obtain desired ferroelectric polarizability. Reaction and crystallization to form polycrystalline ferroelectric layered superlattice material invariably requires one or more steps of heating the solid film.
A well-known category of techniques for depositing MOD precursors on integrated circuit substrates is chemical vapor deposition (xe2x80x9cCVDxe2x80x9d). CVD techniques are well-suited for small geometries and for achieving highly conformal structures in integrated circuits. In a CVD process, a gaseous stream containing metal organic precursor compounds is brought into contact with the heated surface of the substrate, which causes the metal organic precursor compounds to decompose at the substrate surface and form a solid film of metal-containing compounds.
In typical MOD-CVD fabrication of layered superlattice materials, reaction and crystallization of the deposited metal compounds to produce desired electronic properties requires heat treatments in oxygen gas at elevated temperatures. This heating is commonly referred to as annealing. Annealing at elevated temperature causes reaction of the deposited metal compounds and crystallization to form polycrystalline layered superlattice material. The annealing steps in the presence of oxygen are typically performed at a temperature in the range of 800xc2x0 C. to 900xc2x0 C. for 30 minutes to two hours. As a result of the presence of reactive oxygen at elevated temperatures, numerous defects, such as dangling bonds, are generated in the single crystal structure of the semiconductor substrate, leading to deterioration in its electronic characteristics. Annealing at elevated temperature for extended time may also damage other components of the integrated circuit, for example, diffusion barrier layers, which begin to degrade at temperatures of 700xc2x0 C. and above.
The polarizability of ferroelectric layered superlattice material is affected by, among other factors, the grain size of the numerous crystal grains in the polycrystalline ferroelectric material. In the layered superlattice materials, it is believed that relatively large grain size enhances polarizability. Thin films of layered superlattice material formed using conventional CVD techniques, however, tend to have smaller grain size, resulting in decreased ferroelectric polarizability.
A persistent problem in dielectric and ferroelectric materials used in integrated circuits is charge dissipation and leakage current. It is believed that a significant part of leakage current travels along the grain boundaries of the crystal grains in polycrystalline dielectric and ferroelectric material.
The present invention provides a chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) method for fabricating ferroelectric layered superlattice materials in ferroelectric integrated circuits that minimizes the time of exposure to oxygen at elevated temperature, thereby reducing heat and oxidation damage. A CVD method in accordance with the invention produces relatively large grain size in the layered superlattice material, thereby enhancing ferroelectric polarizability. Further, a CVD method in accordance with the invention increases the path length of leakage charges in polycrystalline layered superlattice material, thereby reducing overall leakage current.
The layered superlattice materials, also referred to in the art as layered perovskite-like structures, are represented by the general formula:
A1w1+a1A2w2+a2 . . . Ajwj+ajS1x1+s1S2x2+s2 . . . Skxk+skB1y1+b1B2y2+b2 . . . Blyl+blQzxe2x88x92q,xe2x80x83xe2x80x83(1)
where A1, A2 . . . Aj represent A-site elements in the perovskite-like structure, which may be elements such as strontium, calcium, barium, bismuth, lead, and others; S1, S2 . . . Sk represents superlattice generator elements, which usually is bismuth, but can also be materials such as yttrium, scandium, lanthanum, antimony, chromium, thallium, and other elements with a valence of +3; B1, B2 . . . B1 represent B-site elements in the perovskite-like structure, which may be elements such as titanium, tantalum, hafnium, tungsten, niobium, zirconium, and other elements; and Q represents an anion, which generally is oxygen but may also be other elements, such as fluorine, chlorine and hybrids of these elements, such as the oxyfluorides, the oxychlorides, etc. The superscripts in Formula (1) indicate the valences of the respective elements. For example, if Q is O for oxygen, then q is 2. The subscripts indicate the number of moles of the material in a mole of the compound, or in terms of the unit cell, the number of atoms of the element, on the average, in the unit cell. The subscripts can be integer or fractional. That is, Formula (1) includes the cases where the unit cell may vary throughout the crystalline material, e.g. in SrBi2(Ta0.75N0.25)2O9, on the average, 75% of the B-sites are occupied by a tantalum atom and 25% of the B-sites are occupied by a niobium atom. If there is only one A-site element in the compound, then it is represented by the xe2x80x9cA1xe2x80x9d element and w2 . . . wj all equal zero. If there is only one B-site element in the compound, then it is represented by the xe2x80x9cB1xe2x80x9d element, and y2 . . . y1 all equal zero, and similarly for the superlattice generator elements. The usual case is that there is one A-site element, one superlattice generator element, and one or two B-site elements, although Formula (1) is written in the more general form since the invention is intended to include the cases where the A- and B-sites and the superlattice generator sites of the crystalline material can be occupied by more than one type of atom.
The value of z is found from the equation:
(a1w1+a2w2 . . .+ajwj)+(s1x1+s2x2 . . .+skxk)+(b1y1+b2y2 . . .+b1y1)=qz.xe2x80x83xe2x80x83(2)
Formula (1) includes all three of the Smolenskii type compounds discussed in U.S. Pat. No. 5,519,234, issued May 21, 1996, to Paz de Araujo et al. The layered superlattice materials do not include every material that can be fit into the Formula (1), but only those which spontaneously form themselves into crystalline structures with distinct alternating layers. For example, the layered superlattice material strontium bismuth tantalate, represented by the stoichiometric formula SrBi2Ta2O9, may be viewed as crystalline layers of bismuth oxide alternating with perovskite-like layers containing strontium and tantalum.
Embodiments of a method in accordance with the invention generally provide for flowing a first reactant gas into a CVD reaction chamber containing a heated integrated circuit substrate. The first reactant gas contains a first precursor compound or a plurality of first precursor compounds, and the first precursor compound or compounds decompose in the CVD reaction chamber to deposit a coating containing metal atoms on the heated integrated circuit substrate. Thereafter, a second reactant gas is flowed into a CVD reaction chamber containing the heated substrate. The second reactant gas contains a second precursor compound or a plurality of second precursor compounds, which decompose in the CVD reaction chamber to deposit more metal atoms on the substrate. Heat for reaction and crystallization of the deposited metal atoms is provided by heating the substrate during CVD deposition, as well as by selected rapid thermal processing (xe2x80x9cRTPxe2x80x9d) and furnace annealing steps. The temperature of the heated substrate and the sequence and temperature of the treating steps may vary in the general embodiments of a method in accordance with the invention.
In a first generalized embodiment, the first reactant gas decomposes and a seed layer is deposited on the heated substrate. The temperature of the heated substrate is at a first substrate temperature in the range of from 400xc2x0 C. to 700xc2x0 C. The seed layer comprises metal atoms, usually in the form of metal-oxide compounds; however, these metal oxides are not necessarily the desired layered superlattice material compound. In this embodiment, it is typically necessary to treat the seed layer using RTP for reaction and crystallization of the deposited metal oxides to occur and to form the desired layered superlattice material in the seed layer. Treating of the seed layer may also include a furnace anneal. The seed layer is typically very thin, in the range of from 5 nm to 40 nm, preferably about 25 nm. Therefore, reaction and crystallization of the deposited metal compounds of the seed layer by RTP to form crystalline layered superlattice material is faster and more complete than in thicker films having thicknesses in the range of from 100 nm to 400 nm. Optional treating of the seed layer by heating in a furnace annealing step enhances desired crystallization of the layered superlattice material in the seed layer.
In accordance with the invention, when the second reactant gas reacts in a CVD reaction chamber to deposit a second coating of metal oxide on the seed layer, the presence of crystals in the seed layer enhances the formation of a thin film of polycrystalline layered superlattice material. The presence of the seed layer makes it possible to form polycrystalline layered superlattice material using less exposure to heating at elevated temperatures than necessary with conventional CVD techniques, while obtaining large grain size and good electronic properties. The heated substrate alone at a second substrate temperature in the range of from 400xc2x0 C. to 700xc2x0 C. usually provides sufficient heating for reaction and crystallization of the metal atoms deposited in the second coating to form the final thin film of layered superlattice material. Additional treatment of the substrate by RTP and furnace anneal may be conducted, however, after deposition of the second coating.
In the first embodiment, the seed layer is typically a fairly continuous and uniformly thick layer, preferably about 25 nm thick. In a second general embodiment, the seed layer formed by the first reactant gas may, however, comprise xe2x80x9cislandsxe2x80x9d of layered superlattice material, rather than a continuous layer. Based on deposition rates known from depositing thicker layers, an amount of first precursor compounds corresponding to a layer having a xe2x80x9cnominal thicknessxe2x80x9d in the range of from 5 nm to 25 nm, preferably about 10 nm, is flowed into a CVD reaction chamber. Because the amounts are so small, uneven deposits of metal-containing compounds are deposited as islands of material on the substrate, instead of a smooth, uniform coating. The thickness is referred to as a xe2x80x9cnominal thicknessxe2x80x9d because its thickness is uneven and not practically measurable. The volume of the deposited seed layer corresponds to an amount deposited in a given time period at a known average rate of deposition, even though the amount is not evenly distributed. At a first substrate temperature in the range of from 600xc2x0 C. to 700xc2x0 C., the heated substrate alone usually provides sufficient heating for reaction and crystallization of the metal compounds initially deposited in the islands, forming layered superlattice material. The optional treatment by RTP and by furnace anneal enhances formation of islands of crystalline layered superlattice material in the seed layer. During flowing of the second reactant gas into the CVD reaction chamber, the substrate is heated at a second substrate temperature that is from 10xc2x0 C. to 100xc2x0 C. lower than the first substrate temperature. When the second reactant gas decomposes and deposits metal compounds on the heated substrate, crystalline growth occurs only at the crystalline island surfaces of the seed layer, resulting in large grain size in the final thin film of polycrystalline layered superlattice material. In this embodiment, no treating of the seed layer is required before flowing the second reactant gas. Treating by RTP or furnace anneal may optionally be conducted before or after reaction of the second reactive gas to enhance formation of layered superlattice material.
In the first and second embodiments discussed already, the first reactant gas and second reactant gas usually have identical or similar compositions, containing the same precursor compounds for forming the desired thin film of layered superlattice material. The first reactant gas may, however, contain only superlattice-generator metals, usually bismuth, so that the seed layer or islands of crystalline material formed in the seed layer contain bismuth oxide only. In these variations also, the presence of the crystallized seed layer of bismuth oxide functions to enhance the reaction and crystallization of the metal compounds deposited from the second reactant gas.
In a third generalized embodiment of a method in accordance with the invention, a coating of metal atoms is deposited using CVD by flowing a first reactant gas and a second reactant gas into a CVD reaction chamber containing a heated substrate. The first reactant gas contains one or more precursor compounds containing superlattice-generator-elements, and the second reactant gas contains one or more precursor compounds containing perovskite-like-layer forming elements, that is, the A- and B-site elements in Formula 1, above. The temperature of the heated substrate is in the range of from 400xc2x0 C. to 700xc2x0 C., preferably in the range of from 600xc2x0 C. to 700xc2x0 C. The first reactant gas and second reactant gas are flowed into the CDV reaction chamber for a first predetermined time period, sufficient to deposit an initial coating on the substrate having a thickness or nominal thickness in the range of from 5 nm to 40 nm. The first predetermined time period is based on the average rate of deposition known from previous depositions of layers with a measured thickness. The first predetermined time period is typically in a range of from 10 seconds to 10 minutes, usually 30 seconds to 60 seconds. Then the flow of the second reactant gas, containing perovskite-like-layer-forming atoms, is interrupted for a second predetermined time period, sufficient to deposit a coating containing a metal compound containing atoms of a superlattice-generator element, but no A- or B-site elements. The second predetermined time period is also based on an average rate of deposition known from previous depositions of layers with a measured thickness. The first predetermined time period and the second predetermined time period are usually in the range of from 10 seconds to 10 minutes, preferably in the range of from 30 seconds to 60 seconds. After the second predetermined time period, the flow of the second reactant gas resumes for a third predetermined time period, preferably 30 seconds to 60 seconds. The sequence may be repeated any number of times until the aggregate of the sequential coatings reaches the desired thickness of the thin film. When the temperature of the heated substrate is higher, in the range of from 600xc2x0 C. to 700xc2x0 C., then the temperature is usually sufficient for the metal oxides in the coatings to react and to crystallize, forming the desired layered superlattice material. Optional treating of the seed layer by heating in an annealing step, such as with RTP or furnace annealing, enhances reaction and crystallization. The temperature of the heated substrate during CVD deposition during the various time periods may also be in a lower range of from 400xc2x0 C. to 600xc2x0 C. In this variation of an embodiment in accordance with the invention, it is typically advantageous to treat the deposited aggregate coating using RTP for reaction and crystallization of the deposited metal oxides to occur and to form the desired thin film of layered superlattice material. Treating of the final thin film may also include a furnace anneal.
Typically, in this third general embodiment, the superlattice-generator element in the first reactant gas is bismuth. By interrupting the flow of the second reactant gas, which contains perovskite-like-layer forming atoms, and continuing the flow of the first reactant gas into the CVD reaction chamber, grain boundaries between individual crystal grains are increased in volume and become more circuitous. The leakage path of leakage current thereby becomes longer, resulting in reduced leakage current through the ferroelectric or dielectric thin film. A variation of this embodiment further includes interrupting the flow also of the first reactant gas during the second predetermined time period, and then resuming flow of the first reactant gas at the end of the second predetermined time period.
It is a feature of the invention that thin films of layered superlattice material fabricated by methods in accordance with the invention have improved electronic properties, such as increased ferroelectric polarizability and reduced leakage current, even though the physical crystalline structure of the thin film may not be easily distinguishable from thin films of layered superlattice material fabricated using conventional methods of the prior art. The invention includes, therefore, integrated circuits containing thin films of layered superlattice material fabricated using a method in accordance with the invention, as well as the methods disclosed.